Newbie with jittery servos
« on: April 04, 2018, 10:00:32 am »
Hi, all...

   My goal here is to get familiar enough with Librepilot to successfully set up a couple of 450-size helis with Revo Mini's.  After a lot of reading and searching, I decided to improve my chances by starting out with a very simple setup, to gain familiarization with the GCS.   Here's my setup:
-- nothing installed in a vehicle, all the following is via pure bench testing.
-- Hitec Aurora 9 Tx
-- Delta8 PPM Rx
-- Revo Mini with a home-brew external adapter board to convert the Revo Mini's 1-wire PWM outputs to 3-wire servo connectors
-- Hobbywing Platinum 50 ESC with 7 Amp BEC
-- No separate PDB
-- Hyperion HS2220-06 motor (probably not relevant)
-- 3 Hyperion DS09 digital servos, with GCS servo output set to 270 Hz.
-- Using LP 16.09 with Revo Mini updated to match

To keep it simple, I went thru the Setup Wizard for a simple fixed-wing.  Everything worked as expected, Status panel shows all blocks are either green or black, motor responds properly to Arming and throttle stick movement.  BUT:  the following two conditions bother me, and I don't know where to start investigating:
1.  The yaw servo exhibits limited movement on one side of neutral.  Swapping servos makes no difference.  I'll pursue this later after more reading.
2.  Servo movements resulting from Tx stick movement are nice and smooth, but if I pick up the Revo Mini and slowly tilt it, the resulting servo movements are very jittery and erratic, with limited range of movement.  As soon as I set the Revo Mini back down, the servos return to neutral and settle down nicely.

   I've not touched any non-Wizard settings in GCS, and the servos are new and known to work perfectly in a non-LP environment.  The only changes I've made were to set up the Revo Mini for external LED (didn't work at all, maybe not supported with the Mini?) and to set the ESC's BEC output to 6V instead of 5V.  The Mini was upgraded to 16.09 by using the erase option so most GCS settings should still be defaulted.

What should I look for to investigate the jittering?


  • *****
  • 4337
    • LibrePilot
Re: Newbie with jittery servos
« Reply #1 on: April 04, 2018, 10:22:01 am »
Please post your config file.

Try using a stabilization settings with all axis: Roll, Pitch, Yaw to Manual and see if the jitter still.

Re: Newbie with jittery servos
« Reply #2 on: April 04, 2018, 06:29:12 pm »
Remove any props before doing this:

If you go to Output page and try "Test Outputs", then the signals will be perfectly stable because they are then controlled by the sliders on that page.  If the servos still jitter, then there is a conflict between the servo protocol/rate you have selected ([email protected]) and what the servos are designed to accept.

Re: Newbie with jittery servos
« Reply #3 on: April 05, 2018, 10:31:18 am »
   Well, I'm back...

To fs5oh:  As I expected, setting the flight mode parms to 'Manual' prevents the servos from moving at all when the Revo Mini is tilted. The servos still followed the Tx stick movements smoothly and properly.  I've tried so many different settings that I'm going to reload the firmware and start over in order to get an 'undiluted' config file to upload.

 To Cliff:  Using the output sliders, the servos (and motor) respond smoothly and properly to the Output slider movements, regardless of whether the outputs are set to 270 Hz or 50 Hz.   270 Hz is Hyperion's recommended setting for their DS09 servos.   I did not try changing 'PWM' to anything else because the other options all appear to be for ESCs, not for servos.  Am I wrong?

I also want to check into the Delta8 Rx itself.  It's a multi-mode receiver that can be bound to either Futaba FHSS Tx's or to Hitec AFHSS Tx's, and can also be bound in either PPM mode or PWM mode, so I suppose it's possible there's an incompatibility  there somewhere when the Revo Mini is driving servos instead of ESCs...  I do know the Delta8 works as expected with quads.  I was going to use a Hitec Optima D (SBUS only) Rx, known to work well with the Aurora 9 Tx, but I'll have to wait for it to come back from Hitec warranty service some time next week, and then make up a new Rx-to-Revo cable.

To be continued...


  • *****
  • 4337
    • LibrePilot
Re: Newbie with jittery servos
« Reply #4 on: April 05, 2018, 08:35:42 pm »
So all fine about the output config, those DS9 will support at least 400Hz rate and also higher...

Now you may try configuring all the stabilization using Rate,Rate,Axislock (or manual) and see what happens.
Jittering servo may comes from a too high P (outer loop) value while using Attitude.

Attitude response can be adjusted in Stabilization tab (55deg by default)

Re: Newbie with jittery servos
« Reply #5 on: April 08, 2018, 03:34:21 am »
Results of further bench testing:

1.  First, I reprogrammed the ESC to its default values so that its BEC is set to 5V output instead of 6V.  Result:  No changes.. servos still jitter whenh Revo is tilted in any direction, but respond directly to Tx sticks.

2.  Next:  I replaced the Revo Mini itself with another identical unit (also brand new).  Result:  no change.. Servos still follow Tx sticks perfectly, but still jitter when the Revo is slowly tilted in any direction.

2.  Replaced the PPM receiver with a Hiitec Optima (PWM only, known to work perfectly) and changed all PPM settings in GCS from PPM to PWM.  Re-did the Input screen values.  Result:  No change. Still jittery when the Revo is tilted.

3.  Activated a 5th channel for selecting flight modes.  Set first mode to Attitude, second mode to Rate, third mode to Manual.  Result:  As before, servos do not move when Revo is tilted in Manual mode, but still jitter in Attitude and Rate modes.

4.  Gave up for now, put everything back as it was, and did a UAV Settings Export.  I looked it over (first time ever) and immediately noticed something odd:
            <field name="ChannelType" values="PWM,PWM,PWM,PWM,PWM,PWM,PWM,PWM,PWM,PWM,PWM,PWM"/>

...even though I'm back to using the Delta8 PPM receiver.  Is this normal?  I double-checked all related GCS settings...
-- Configuration/Hardware shows Rx Input port set to 'PPM'
-- Configuration/Input shows all active channels set to 'PPM'
-- Configuration/Output shows that channel assignments match Input channels

So..  I've attached the Export file.


  • *****
  • 4337
    • LibrePilot
Re: Newbie with jittery servos
« Reply #6 on: April 08, 2018, 10:08:40 am »
Loaded your config file, all seems fine. (even with a DS9 connected)
While moving the board, is you add vibrations or small shaking by hand to the board you should expect jittering.

<field name="ChannelType" values="PWM,PWM,PWM,PWM,PWM,PWM,PWM,PWM,PWM,PWM,PWM,PWM"/>
This one is related to Outputs, look the UAVO name some lines before: ActuatorSettings

Re: Newbie with jittery servos
« Reply #7 on: April 19, 2018, 06:46:28 pm »
   This servo jitter problem turned out to be due to my using a simple fixed-wing setup as a learning procedure using the Setup Wizard.  I scrapped that approach, and instead, went through the setup procedures for heli, as recommended by Jerry Cronus and John Gera.  Each of those covers some important points for beginners that other does not.

   Anyway, after following those procedures, the jitter disappeared.  I believe the change was due to properly setting the Gyro Noise Filter value.  In any case, I now have an almost-working heli setup with no jitters.

...Back to the LP documentation to research the next steps.  Thanks to all for the help!